Conventional holographic memory systems normally employ a page-by-page storage approach. An input device such as SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page), while a detector array such as CCD camera is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-by-page approach. All of these systems, however, suffer from a common drawback that they require the recording of huge quantities of separate holograms in order to fill the memory to its full capacity. A typical page-oriented system using a megabit-sized array would require the recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the hologram exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, another conventional holographic ROM system has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds.
FIG. 1 is a view schematically illustrating a conventional method for recording data on a disc-type holographic medium. As shown in FIG. 1, a data mask 48 is placed above a holographic medium 50 which serves as an optical data storage medium, while a conical mirror 32 is placed below the holographic medium 50. To record data on the holographic medium 50, a signal beam is irradiated downward onto the upper surface of the holographic medium 50 via a bit pattern of the data mask 48 and at the same time, a reference beam is irradiated onto the lower surface of the holographic medium 50 after the reflection by the conical mirror 32. The signal beam is interfered with the reference beam at the holographic medium 50, thereby recording the holographic data on the holographic medium 50 according to the bit pattern of the data mask 48.
When conical mirrors having different base angles are used, it is possible to record a plurality of holographic data in the same physical space of the holographic medium 50 by angular multiplexing (see “Holographic disk recording system”, U.S. patent application publication No. US2003/0161246A1, by Ernest Chuang, et al.).
FIG. 2 shows a conventional holographic ROM system including a light source 10; a shutter 12; mirrors 14, 28, 34, 40; HWPs (half wave plates) 16, 24, 36; spatial filters 18, 30, 42; lenses 20, 44; a PBS (polarization polarization beam splitter) 22; polarizers 26, 38; a conical mirror 32; a data mask 48; and a holographic medium 50.
The light source 10 emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam, which is of only one type of linear polarization, e.g., P-polarization or S-polarization, is provided to the mirror 14 via the shutter 12 which is opened to transmit the laser beam therethrough when recording data on the holographic medium 50. The mirror 14 reflects the laser beam to the HWP 16. The HWP 16 rotates the polarization of the laser beam by θ degree (preferably 45°). And then, the polarization-rotated laser beam is fed to the spatial filter 18 for removing noises included in the polarization-rotated laser beam. And then, the polarization-rotated laser beam is provided to the lens 20 for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS 22.
The PBS 22, which is manufactured by repeatedly depositing at least two kinds of materials each having a different refractive index, serves to transmit, e.g., a horizontally polarized laser beam, i.e., P-polarized beam, along a S1 path and reflect, e.g., a vertically polarized laser beam, i.e., S-polarized beam, along a S2 path. Thus the PBS 22 divides the expanded laser beam into a transmitted laser beam (hereinafter called a reference beam) and a reflected laser beam (hereinafter called a signal beam) having different polarizations, respectively.
The signal beam, e.g., of a S-polarization, is reflected by the mirror 34. And then the reflected signal beam is provided to the mirror 40 via the HWP 36 and the polarizer 38 sequentially. Since the HWP 36 can rotate the polarization of the signal beam by θ′ degree and the polarizer 38 serves to pass only a P-polarized signal beam, the HWP 36 and the polarizer 38 can regulate the amount of the P-polarized signal beam arriving at the mirror 40 by changing θ′. And then the P-polarized signal beam is reflected by the mirror 40 toward the spatial filter 42 for removing spatial noise of the signal beam and allowing a Gaussian beam thereof to be transmitted therethrough. And then the signal beam which is a perfect Gaussian is provided to the lens 44 for expanding the beam size of the signal beam up to a preset size. Thereafter, the expanded signal beam is projected onto the holographic medium 50 via the data mask 48. The data mask 48, presenting data patterns for recording, functions as an input device, e.g., a spatial light modulator (SLM).
Meanwhile, the reference beam is fed to the mirror 28 via the HWP 24 and the polarizer 26 sequentially. Since the HWP 24 can rotate the polarization of the reference beam by θ″ degree and the polarizer 26 serves to pass only a P-polarized reference beam, the HWP 24 and the polarizer 26 can regulate the amount of the P-polarized reference beam arriving at the mirror 28 by changing θ″. Therefore, the polarization of the reference beam becomes identical to that of the signal beam. And then the mirror 28 reflects the P-polarized reference beam toward the spatial filter 30 which removes spatial noise of the signal beam and allows a Gaussian beam thereof to be transmitted therethrough. And then the reference beam which is a perfect Gaussian beam is projected onto the conical mirror 32 (the conical mirror 32 being of a circular cone having a circular base with a preset base angle between the circular base and the cone), which is fixed by a holder (not shown). The conical mirror 32 reflects the reference beam toward the holographic medium 50. The incident angle of the reflected reference beam on the holographic medium 50 is determined by the base angle of the conical mirror 32.
When conical mirrors having different base angles are used in the above-mentioned holographic data recording apparatus, it is possible to record holographic data in the same physical space of the holographic medium 50 by angular multiplexing. In other words, another conical mirror having a base angle different from the base angle of the conical mirror 32 is used in the holographic data recording apparatus, in place of the conical mirror 32, the incident angle of the reference beam irradiated onto the holographic medium 50 is changed so that the reference beam and the signal beam provide a new interference pattern. Thus, new holographic data can be recorded on the holographic medium 50 by angular multiplexing.
However, the conventional holographic data recording apparatus is problematic in that, in order to reflect a reference beam toward a disc-type holographic medium at a desired incident angle, it is necessary to use a conical mirror having a specified base angle capable of providing the desired incident angle. Thus, to record a plurality of holographic data on a holographic medium by angular multiplexing, the required number of conical mirrors must be the same as the desired number of the incident angles of the reference beam, so that the cost of the holographic data recording apparatus is increased.
Furthermore, the replacement of every conical mirror is a difficult and complex process so that the recording speed is decreased.